The success of a well-drilling operation depends on many factors, none of which is more important than the drilling fluid or mud. Drilling fluids perform a variety of functions which influence the drilling rate, cost, efficiency and safety of the operation. More specifically, drilling muds prevent the influx of formation fluids into the wellbore, seal exposed permeable formations to prevent leakage of the drilling fluid into the formation, maintain the stability of exposed formulations, cool and lubricate the bit and drill string, hold back pressure and stabilize the formation, e.g., shale inhibition. How well the drilling fluid fulfills these requirements greatly affects the cost of the operation and the productivity of the well.
During operation, drilling fluids are pumped down a hollow drill string, through nozzles in the bit at the bottom of the well, and back up the annulus formed by the hole or casing and drill string to the surface. Once reaching the surface, the drilling fluid is passed through a series of vibrating screens, settling tanks, hydrocyclones and centrifuges to remove formation debris brought to the surface. It is thereafter treated with additives to obtain the desired set of properties; pumped back into the well and the cycle is repeated.
Drilling fluids are generally composed of liquids, e.g., water, petroleum oils, synthetic oils and other organic liquids; dissolved inorganic and organic additives; and suspended, finely divided solids of various types. Drilling fluids are classified as to the nature of the continuous phase; thus there are four main divisions: gaseous (including foam), water-base, oil-base, or synthetic. Growing concern among government and environmental agencies over the environmental impact of drilling fluids has led to a significant increase in the industry's reliance on water-based muds. In fact, about 85% of all drilling fluids used today are water-based systems. The types depend on the composition of the water phase (pH, ionic content, etc), viscosity builders (clays, polymers or a combination), filtration control agents (clays, polymers or a combination) and other rheological control agents (deflocculants or dispersants (qv)). Generally, there are six main categories or types of water-based muds:
Fresh Water.
Fresh water fluids range from clear water having no additives to high density muds containing clays, barite, and various organic additives. The composition of the mud is determined by the type of formation to be drilled. When a viscous fluid is required, clays and/or water-soluble polymers (qv) are added. Fresh water is ideal for formulating stable drilling fluids as many mud additives are most effective in a system of low ionic strength. Inorganic and/or organic additives control the rheological behavior of the clays, particularly at elevated temperatures. Water swellable and water soluble polymers and/or clays may be used for filtration control. Mud pH is generally alkaline and, in fact, viscosity control agents like the montmorillonite clays are more efficient at a pH&gt;9. Sodium hydroxide is by far the most widely used alkalinity control agent. Freshwater muds can be weighted with insoluble agents to desired density required to control formation pressures.
Seawater.
Many offshore wells are drilled using a seawater system because of ready availability. Seawater muds generally are formulated and maintained in the same way that a freshwater mud is used. However, because of the presence of dissolved salts in seawater, more electrolyte stable additives are needed to achieve the desired flow and filtration (qv) properties.
Salt Water.
In many drilling areas both onshore and offshore, salt beds or salt domes are penetrated. Saturated salt muds are used to reduce the hole enlargement that would result from formation-salt dissolution by contact with an undersaturated liquid. In the United States, the salt formations are primarily made up of sodium chloride. In other areas, e.g., northern Europe, the salt may be composed of mixed salts, predominantly magnesium and potassium chlorides. It has become quite common to use high (20-23 wt % NaCl) salt muds in wells being drilled in deep (&gt;500-m water depth) water regions of the Gulf of Mexico. The reasons are twofold: stabilization of water-sensitive shales and inhibition of the formation of gas hydrates. The high salinity of salt water muds may require different clays and organic additives than those used in fresh- or seawater muds. Salt water clays and organic polymers contribute to viscosity. Filtration properties are adjusted using starch (qv) or cellulosic polymers. The pH ranges from that of the makeup brine which may be somewhat acidic, to 9-11 through use of sodium hydroxide or lime.
Calcium Treated.
Fresh- or seawater muds may be treated with gypsum or lime to alleviate drilling problems that may arise from drilling water-sensitive shale or clay-bearing formations. Gyp muds (gypsum added) are generally maintained at a pH of 9-10, whereas lime muds (lime added) are in the 12-13 pH range. Calcium-treated muds generally require more additives to control flow and filtration properties than those without gypsum or lime.
Potassium Treated.
Generally potassium treated systems combine one or more polymers and a potassium ion source, primarily potassium chloride, in order to prevent problems associated with drilling certain water-sensitive shales. The flow and filtration properties may be quite different from those of the other water-base fluids. Potassium muds have been applied in most active drilling regions around the world. Environmental regulations in the United States have limited the use of potassium muds in offshore drilling owing to the apparent toxicity of high potassium levels in the bioassay test required by discharge permits.
Low Solids.
Fresh water, clay, and polymers for viscosity enhancement and filtration control make up low solid and so called non-dispersed polymer muds. Low solids muds are maintained using minimal amounts of clay and require removal of all but modest quantities of drill solids. Low solid muds can be weighted to high densities, but are used primarily in the unweighted state. The main advantage of these systems is the high drilling rate that can be achieved because of the lower colloidal solids content. Polymers are used in these systems to provide the desired rheology, especially xanthan has proven to be an effective solids suspending agent. These low solid muds are normally applied in hard formations where increasing the penetration rate can reduce drilling costs significantly and the tendency for solids buildup is minimal.
Bentonite is by far the most commonly used clay in drilling muds because it provides excellent rheological and filtration properties to the mud, especially in combination with polyelectrolytes like CMC. Bentonite clay which mainly is montmorillonite (a smectite type of clay) exists of very thin platelets (sheets). A number of attempts have been made to determine the particle size of sodium montmorillonite but this is rather difficult because of the flat, thin irregular shape of the platelets and because of the wide range of sizes. The clay platelets exhibit a superior ability to swell uniformly in fresh water upon shear application. The swelling of the dehydrated agglomerated bentonite clay when it is contacted with water is caused by a penetration of water molecules in between the clay platelets. The swelling pressure is so strong that the layers separate into smaller aggregates and even into individual unit layers with a thickness of 10 .ANG.. Thus, a relatively stable suspension of the hydrated clay can be obtained.
In aqueous suspensions, the edges of the bentonite platelets are positively charged while the faces are negatively charged. Because of these opposite charges there is an interaction with the positive edges and negative faces. However in a fresh water hydrated bentonite suspension (without electrolytes) these electrostatic interactions are rather weak because of the thick bounded water layer around the clay platelets. This thick water layer keeps the particles that far from each other that the bentonite is almost completely dispersed but still a very weak flocculation remains as is shown by the gel properties and yield stress.
A wide variety of organic polymers also serve a number of useful purposes in drilling fluids such as increasing viscosity and controlling filtration rates, which often are directly related to the degree of flocculation and aggregation of the bentonite clay particles in the drilling mud. The ability to reduce fluid loss is also influenced by these properties, i.e., in order to build up a good filtercake to minimize filtrate loss into the formation, the clay suspension should be in a deflocculated condition. These polymers are either natural polysaccharides, e.g., starch, guar gum, xanthan gum, and other biopolymers; or derivatives of natural polymers, e.g., derivatives of cellulose, starch, guar and other biopolymers; or lignosulfonate, lignite and synthetic polymers, e.g., polymers and copolymers of acrylic acid, acrylonitrile, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). The most commonly used polymeric viscosity builders are the cellulosics, starches, xanthan gum, and polyacrylamides.
Sodium carboxymethyl cellulose (CMC) and polyanionic cellulose(PAC) are two of the more widely used anionic polymers in drilling fluids in order to control viscosity and filtration rates. The effectiveness of CMC, being a polyelectrolyte, as a viscosity builder has its limitations, however, as its effectiveness decreases with increasing electrolyte concentration. In fresh water low DS CMC's adsorb on bentonite while higher DS CMC's (e.g. PAC's) shows a decreased amount of adsorption. Only a very small amount of a low DS CMC (DS--0.7) is sufficient to realize complete dispersion of the bentonite as it adsorbs on the positive edges of the platelets. This complete dispersion results in a reduction of the gel-strength to almost zero. In general a good dispersed bentonite/CMC suspension gives a good build-up of the filter-cake and an excellent fluid loss reduction performance is obtained. Such a system does not, however, demonstrate significant gel-strength and yield point.
It is clear that the rheological properties of a drilling fluid determines very much the success of the overall drilling operation. The Theological properties of the mud determine very much (1) the hole cleaning efficiency and hole stability, (2) cuttings suspension efficiency, (3) mud hydraulics performance, (4) ease of mud handling operations, and (5) rate of penetration. The rheological requirements for these diverse purposes may often conflict, so it is necessary to optimize the mud properties in order to obtain the best overall performance. Optimal rheological properties are necessary in order to carry the drilled cuttings efficiently to the surface. While drilling, the high shear viscosity should not be too high to allow an efficient transmission of the hydraulic horsepower of the drilling fluid to the drill bit. But when the circulation is slow or interupted the viscosity should be high and the gel-strength should be sufficient to prevent/minimize settling of the cuttings. Such rheology profile can be obtained by using a thixotropic system, which in the case of a bentonite system can be realized with a somewhat flocculated system at which the particle links are temporarily broken by/during stirring and are only restored during rest. On the other hand a the drilling fluid should cover the wall of the borehole with a thin filtercake in order to stabilize the borehole and to prevent loss of the circulation fluid into the drilled porous formation. In general this is done most efficiently by a well dispersed clay suspension.
The rheology and thus the flow properties of a drilling mud is influenced by:
1. The state of hydration and dispersion of bentonite or other clay particles in the aqueous phase and the amount of them. PA0 2. The rheology of the aqueous phase as it might be modified by additives (viscosifiers) e.g. by the polymers mentioned earlier. PA0 3. The clay interparticle forces as well as by the interaction of the polymers with the clay and/or with each other. PA0 a pseudoplastic rheology behaviour PA0 a sufficient Yield Point and gel-strength with a fast build up of the gel-strength in the first 10 sec PA0 a low to moderate Plastic Viscosity (PV) PA0 sufficient fluid loss reduction PA0 3-chloro-2-hydroxypropyl dimethyidodecyl ammonium chloride; 3-chloro-2-hydroxypropyl dimethyloctadecyl ammonium chloride; 3-chloro-2-hydroxypropyl-dimethyloctyl ammonium chloride; 3-chloro-2-hydroxypropyl trimethyl ammonium chloride; 2-chloroethyl trimethyl ammonium chloride; and the like.
Thus, the solids content is related to the mud rheology as well as to the mud density. And in general the higher the solid content of a mud (added clays and drilled cuttings which might become dispersed) the more difficult and time consuming the mud cleaning will be. In this respect low-solid mud systems can improve the rate of penetration.To maintain the desired rheological profile for such low-solid mud systems viscosifying polymers are added. Examples of such polymers are xanthan, high molecular weight CMC's and acrylic polymers.
The claimed invention relates to the use of quaternary nitrogen containing amphoteric water-soluble polymers which proved to be very efficient drilling fluid rheology modifiers especially for bentonite containing system while it allows very low uses of bentonite and provides the desired drilling fluid characteristics. The additives of the invention improve the overall performance of the mud by enhancing the following properties: